Dienes, especially isoprene, are useful as monomers for the manufacture of synthetic rubbers. Isoprene is primarily used to make cis-polyisoprene which is a stereospecific rubber having the same segmeric unit as natural rubber. Several fundamental processes have been used to construct the isoprene C.sub.5 skeleton from smaller carbon units. These processes are not commercially accepted in that there are numerous problems associated with each particular synthesis route. One route involves condensing acetylene and acetone followed by hydrogenation and dehydration. Another route involves as a first step the reaction between formaldehyde and isobutylene, and in a subsequent step the intermediate derivative is catalytically cracked at elevated temperatures. See for example, French Pat. No. 1,294,716; Chem. Abstracts 57:15309.
European Patent Application No. 80449 based on U.S. application Ser. No. 315,803 discloses the synthesis of isoprene from linear butenes wherein mixed linear butenes are catalytically isomerized to a mixture of cis- and trans- butene-2, and then hydroformylating the butene-2 mixture to 2-methylbutanal (2MBA) in the presence of a homogeneous rhodium catalyst and organic ligand. The 2MBA is then dehydrated to isoprene in the presence of acidic heterogeneous catalysts at elevated temperatures. This European patent application discloses a preferred catalyst for the dehydration step as a boron phosphate which is described in British Pat. No. 1,385,348. Commercial production of isoprene via the aldehyde dehydration route has not been established since the dehydration catalyst is known to have short lifetimes which limit its utility in commercial applications.
U.K Pat. No. 1,385,348 relates to the conversion of aldehydes to dienes with conjugated double bonds. This British patent recites that particularly preferred acid dehydration catalysts are mixed acid anhydrides, for example, boron phosphate, silicoborate or silicotitanate.
A disadvantage associated with known catalysts to dehydrate aldehydes is that catalyst life depends on many factors which include catalyst composition and structure, catalyst activity, operating temperatures and coke deposition. Coke deposition is understood to denote coke (carbonaceous) deposits formed on the catalyst during the dehydration reaction. As stated earlier, no commercial process based on said technique has been developed so far, since there is no catalyst with selectivity and stability to justify a commercial process.
The use of boron phosphate as a catalyst for the dehydration of alcohols such as 2-butanol and 2-methyl-2-butanol is known. See Jewur and Moffat, Journal of Catalysis, 57, 167-176 (1979). The problems associated with an aldehyde dehydration are different and more difficult to overcome than those found in alcohol dehydrations. For example, the boron phosphate dehydration of 2-methyl-2-butanol yields only 2-methyl-2-butene and 2-methyl-1-butene, while dehydration of 2MBA yields primarily methylisopropylketone (MIPK), 2-methyl-2-butene, 2-methyl-1-butene and isoprene. It is the production of the conjugated diolefin, isoprene, that makes the aldehyde dehydrations so difficult, since this highly reactive monomer is known to form dimers and/or polymerize in the presence of acid catalysts.
In addition, aldehydes such as 2MBA are known to undergo aldol condensation. This is a reaction between two molecules of an aliphatic aldehyde whereby a 3-hydroxyaldehyde is formed. Dehydration of the 3-hydroxyaldehyde results in the formation of terpenes, a highly undesirable by-product that can coke and deactivate the catalyst. Due to these and other differences, catalysts suitable for long term dehydration of alcohols have not been found acceptable for aldehyde dehydration.
Regarding the prior art of polymer bound catalysis, there is a general belief by those skilled in the art that all types of polystyrene resins (macroreticular or gel) are inherently thermally unstable both in the presence or absence of oxygen. The upper temperature limit often cited for use of these catalyst-resin systems is quoted at approximately 150.degree. C. See Sherrington "Polymer Supported Reactions in Organic Synthesis"; Chap. 1 p. 27; Wiley: N.Y., 1980; See also: International Workshop on Heterophase Attached Homogeneous Catalysis, Grenoble, France, 1977 (CNRS and NSF) and Chauvin et al, "Polymer Supported Catalysts" Prog. Polymer Sci., Vol. 5, p. 100, Pergamon Press, (1977). The present innovation is concerned with functionalized macroreticular polystyrene that has utility as a catalyst for vapor phase dehydration reactions at temperatures in excess of 200.degree. C.
Substituted phosphines have been used to chemically link a catalyst metal to a polymer support. Examples of this are found in Grubbs et al, "Polymer Preprints," American Chemical Society, Division Polymer Chemistry, 1972, Vol. 13, No. 2, pages 828-832 [Chem. Abs. Vol. 81, 6555d (1974)] and also Grubbs et al, "J. Macromol. Sci Chem.," 1973, Vol. 7, No. 5, Pages 1047-1063, [Chem. Abs. Vol. 78, 164622r (1973)].
U.S. Pat. No. 4,230,633 discloses polymer supported metal complexes wherein the ligand is a cycloalkadienyl radical with metals from Group VIII of the Periodic Table.
U.S. Pat. No. 4,292,415 discloses a crosslinked polystyrene with cycloalkadienyl ligands and Group VIII metal carbonyls.
U.S. Pat. No. 4,323,698 discloses a weak base anion exchange resin which has been contacted with a solution of a coordination compound having at least two ligands connected to at least one central metal atom, to chemically bond the resin to the metal atom by replacement of at least one of the ligands of the coordination compound by a functional group of the weak base anion exchange resin. The complex can be used as a catalyst for hydrogenation, carbon monoxide insertion, polymerization, isomerization, vinyl ester exchange and ethylene oxidation reactions among others.
U.S. Pat. No. 4,144,191 discloses amine resins loaded with bimetallic clusters as novel hydroformylation catalysts. This patent is directed to the conversion of liquid olefins to alcohols in a one-step hydroformylation process which consists of contacting an olefin, such as 1-hexene in the liquid phase, with a gaseous mixture of carbon monoxide and hydrogen and the presence of a catalyst prepared by loading a bimetallic cluster onto an amine resin.
U.S. Pat. No. 4,238,358 discloses the use of anthranilic acid as a ligand for rhodium, palladium, platinum and ruthenium complexes. These catalysts are disclosed as reduction catalysts for liquid phase reactions, i.e. the hydrogenation of olefinic and aromatic hydrocarbons.
The prior art does not disclose or suggest the use of resin phosphorous complex catalysts in vapor phase dehydration reactions. One skilled in this art would readily realize or assume that resins, particularly polystyrene resins would not hold up at the temperatures at which vapor phase dehydration reactions are conducted. See Catalysis, J. R. Anderson and M. Boudant, Eds. Chapter 4; Springer Verlag (1981). One aspect of the present invention is the discovery that the catalysts of the invention can operate at a temperature range from 200.degree. to 300.degree. C.
A portion of the instant invention is directed to a catalyst of high selectivity and low coke deposition in conjunction with extended catalyst lifetimes. The prior art does not suggest or disclose a polymer bound catalyst for the dehydration of aldehydes to dienes which would be suitable for commercial application. The prior art catalyst deactivation can be attributed to coking and possible degradation by water (a by-product of the dehydration). These problems can be limited by binding the catalytic species to a hydrophobic support such as a hydrocarbon polymer or resin.